Presentation Speech by Professor Per-Olov Löwdin of the Royal Academy of Sciences

Translation from the Swedish text

Your Majesties, Your Royal Highnesses, Ladies and Gentlemen,

This year’s Nobel Prize in physics is shared equally between Philip Anderson, Sir Nevill Mott and John Van Vleck for their fundamental contributions to the theory of the electronic structure of magnetic and disordered systems.

All matter consists of positive and negative electricity: partly heavy positive elementary particles gathered in atomic nuclei, partly light negative elementary particles – electrons – which move in wonderful patterns around the nuclei – always attracted to them but difficult to catch because of their own movement. It is this electron dance which is essentially responsible for the electric, magnetic, and chemical properties of matter.

The 1937 Nobel Prize winner in medicine, Albert Szent-Györgyi, has often compared the chemical process in living cells with a great drama played with the electrons as actors on a stage formed by the biomolecules – with the only difference that the scene as well as the actors may be a thousand billion times smaller than we are accustomed to from the Royal Opera. No scientist has seen the score of this musical of life itself, and no one will probably ever be able to see it in its entirety – only a few have been granted the privilege of seeing small fragments in the form of isolated ballets, often with a hero and sometimes with a ballerina.

In the crystal and ligand field theories developed by Van Vleck, there is always a metal atom playing the role of the hero in the drama. In many of the enzymes fundamental for the life of our body, there is often a metal atom in the active center which regulates the action. The haemoglobin in our red blood cells contains an iron atom which carries the oxygen molecule to its given place in the body – in the same way as the hero carries the ballerina on his strong arms. It is Van Vleck who has developed the basic theory for such processes, which are also of great importance in the chemistry of complex compounds, geology, and laser technology.

The electronic dance is of similar importance also in the solid bodies surrounding us – in the ladies’ diamonds, in the every-day rock salt, or in the amorphous glasses. Such materials have characteristic electric and magnetic properties which depend on the motions of the electrons. In the same way as it is easier in an ordinary waltz to waltz forward than backward, there is in the electronic dance a specific spin-orbit coupling between the rotations of the electrons and their translational movements, which is of importance for the magnetic properties. Like the dancers in a ballet are constantly changing place, the electrons have also their own exchange and superexchange phenomena – their own characteristic “pas de deux”. Both Van Vleck and Anderson have studied the local magnetic properties of matter, where the hero is a metal atom with strong personal magnetism whose special properties may vary strongly with the environment – a theory basic for the construction of dilute magnetic alloys. Here one dares perhaps to speak about a successful localization policy.

One of the greatest current problems of humanity is the so-called energy problem – it has been said that the modern society uses too much energy. According to the laws of physics, such a statement is quite absurd, since energy can neither be created nor destroyed. The whole thing is instead a problem of order – at the level of the elementary particles. What happens is that energy of higher order is transformed into energy of lower order, that mechanical and electric energy are changed into heat, that the motions of the elementary particles involved will be more and more disordered. It is the merit of Anderson to have shown that even the reverse may sometimes happen: that geometrically disordered materials, as for instance glass, have their own laws, and that the electronic dance in them may lead to localized states with a high form of order, which influence the properties of the material. Perfectly ordered systems are of great importance in electronics, but they are usually very expensive to produce, so disordered systems with similar properties are hence of essential importance.

In some of his work, Sir Nevill Mott has taken up these and similar ideas in order to study the electrical properties of materials and the transition between conductors, semi-conductors and insulators. In this connection, Mott has also investigated the importance of the interaction between the electrons – that the electrons indeed like to dance in pairs, but also that there is a mutual repulsion which sometimes causes them to guard their own domains and stop the hand-in-hand dance which is essential for the electronic conductivity of the material. The theory for Mott-transitions and Mott-Anderson transitions is today of fundamental importance for the understanding of certain materials and for the construction of new ones. Anderson and Mott have shown that properly controlled disorder may be technically as important as perfect order.

This year’s Laureates in physics are all three giants within solid-state theory, and it is actually rather remarkable how small a portion of their total work has been considered in connection with this year’s Nobel award. Even if these discoveries already now have shown their technical value, it is their fundamental contributions to the free basic research – to the human knowledge of the electronic structure of solids – which has primarily been awarded, with the understanding that it may be even more awarded, with the understanding that it may be even more practically important in the future. Through their work, Anderson, Mott, and Van Vleck have shown that the understanding of the electronic choreography is not only remarkably beautiful from the point of view of science but also of essential importance for the development of the technology of our every-day life.

I have the pleasure and the honour on behalf of the Academy to extend to you our warmest congratulations and I now invite you to receive your prizes from the hands of His Majesty the King.